given V=-3000t+21,000 can be used to find the value V of a car at the end of t years? In how many years is the car worth 12,000?

The value of the car after t years is
\[V(t)=21,000 - 3000t\]
In other words, the car starts off being worth $21,000 and it loses $3,000 in value every year.
If you want to know how many years it takes for the car to reach $12,000, then solve:
\[12000 = 21,000 - 3000t\]
You can do this in your head, because everything is divisible by 1,000. :)
\[12 = 21 - 3t\]
Now just solve for t. :)
Hope this helps!